The joints of the body can be classified as between those that provide immovable articulations (synarthroidal), mixed articulations (amphiarthroidal), and movable articulations (diarthroidal). The ability of amphiarthroidal and diarthroidal joints to provide effective and pain-free articulation, and/or to serve their weight-bearing function, is generally dependent on the presence of intact, healthy fibrocartilage within the joint.
In an amphiarthroidal joint such as the lumbar joint of the back, the vertebra are separated by an intervertebral disc formed of fibrocartilage. More particularly, the intervertebral disc is comprised of an outer annulus fibrosis formed of fibrocartilage. The annulus, in turn, surrounds and contains a more fluid material known as the nucleus pulposus. By virtue of its fluidity, the nucleus allows for both movement and weight-bearing energy transfer. In healthy, generally younger individuals, the annulus is intact and the nucleus pulposus remains quite fluid.
As people age, however, the annulus tends to thicken, desiccate, and become more rigid. The nucleus pulposus, in turn, becomes more viscous and less fluid and sometimes even dehydrates and contracts. The annulus also becomes susceptible to fracturing or fissuring. These fractures tend to occur all around the circumference of the annulus, and can extend from both the outside of the annulus inward, and from the interior outward. Occasionally, a fissure from the outside will meet a fissure from the inside and will result in a complete rent through the annulus fibrosis. In a situation like this, the nucleus pulposus may extrude out through the intervertebral disc. The extruded material, in turn, can impinge on the spinal cord or on the spinal nerve rootlets as they exit through the intervertebral foramen, resulting in the symptoms associated with the classic "ruptured disc".
The current surgical approach to treating a degenerated intervertebral disc generally involves the process of microdiscectomy, in which the site is accessed and the protruded material is removed. This often produces significant relief, provided it is a fairly minor, or mild, localized disc protrusion. In such a procedure a small incision is made, through which the disc is visualized. The area of protruded material is removed, thus decompressing the nerve rootlet that has been impinged on by the extruded material.
In more severe situations, however, the annulus fibrosis becomes degenerated to the point where very little disc space remains, and much of the nucleus pulposus is either contracted or has been extruded. Regional osteophytes can also develop around these areas. The combination of the extruded material and the osteophytes, together with the narrowing of the intervertebral disc space produces a marked narrowing of the intervertebral foramen and impingement on the spinal nerve rootlet as it exits the canal. This is the classical situation that results in radicular pain with axial loading.
When this occurs it becomes necessary to reestablish the intervertebral space. The current approach to this more severe situation is a lumbar laminectomy (to decompress the nerve rootlet) with fusion of the disc space. The bony lamina is removed to decompress the intervertebral foramina and the bone graft is taken from the anterior iliac crest and attached from one vertebrae body to the next. The resulting fusion will maintain stability at that point and also help maintain the separation of the vertebrae.
Recent advances in this technology have been developed by such companies as Spine-Tech, Minneapolis, Minn., which involves the use of a titanium alloy cylinder. The cylinder is screwed into the intervertebral space to assure the stability of the spacing until a fully bony ankylosis can be obtained. The cylinders are packed with bone and are fenestrated so that the packed bone can grow out into the adjacent vertebrae and solidify the fusion. To date, however, clinical results on the long-term follow up of these patients are not available and the efficacy is still in doubt with many spine surgeons.
It would therefore be particularly useful to be able to repair such injuries in a manner that avoids invasive surgical procedures and the problems associated therewith.